This invention relates to potassium hexatitanate fibers useful as a reinforcement for plastics and light alloys. The invention also relates to a process for producing the fibers, as well as a metal-base composite material containing the fibers. More specifically, the invention relates to potassium hexatitanate fibers that at least have an average fiber length of 8-30 .mu.m, an average fiber diameter of 0.2-0.7 .mu.m and a specific surface area of 2-4 m.sup.2 /g, as well as a process for producing the fibers and a metal-base composite material containing them.
The invention also relates to potassium hexatitanate fibers that are low in the contents of impurities, in particular, niobium, and that will provide better performance when used as a reinforcement for a matrix light alloy; the invention further relates to a composite material that comprises the light alloy reinforced with those fibers. The invention also relates to a composite material that is reinforced with at least high-grade potassium hexatitanate whiskers having a total impurity content of no more than 0.3 wt %.
It has heretofore been known that potassium titanate fibers react vigorously with molten aluminum (see H. Fukunaga, M. Takeichi and N. Ohta, J. Jpn. Soc. Compos. mater. 8, 1982, 66). With a view to solving this problem, various attempts have been made and one proposal is potassium hexatitanate fibers having a tunnel structure that have a free potassium content of no more than 5 ppm (see Japanese Patent Public Disclosure No. 301516/1989). Those potassium titanate fibers are characterized in that the content of potassium titanate with a layered structure in potassium hexatitanate fibers having a tunnel structure is reduced to a very low level and the development of such potassium titanate fibers has motivated researchers to pay increasing attention to the potential use of those fibers as a reinforcement for light alloys that are intended to fabricate general-purpose engineering materials (see K. Suganuma, T. Fujita, K. Niihara, T. Okamoto and S. Suzuki; J. Mater. Sci. Letters, 8, 1989, 808).
However, those potassium titanate fibers have a high content of aggregates in the form of bundles ranging from 10 to 30 .mu.m in diameter and, hence, metal-base composite materials reinforced with those fibers experience so great variations in mechanical strength that they have not been considered to be preferred engineering materials (see K. Suganuma et al.; Journal of the Japan Society of Powder and Powder Metallurgy, 8, 1990). Those potassium titanate fibers have the additional disadvantage of high reactivity with the magnesium component of light alloys (see K. Suganuma, T. Fujita, K. Niihara and N. Suzuki; The Proceedings of the 75th Autumun Conference of The Japan Institute of Light Metals, 1988, p.81). A further problem with the potassium titanate fibers is that if metal-base composite materials reinforced with them are heat-treated with a view to improving their mechanical strength and resistance to stress corrosion cracking, the strength is reduced rather than increased.
Furthermore, plastics reinforced with those potassium titanate fibers are not only poor in surface smoothness on account of the presence of the above-described aggregates in the form of bundles; the aggregates will also clog mold gates when miniature parts are to be formed using compounds incorporating those fibers. The term "aggregate" as used herein means a coherent mass of titanate fibers that have clustered together.
As described above, the conventional potassium hexatitanate fibers having a tunnel structure contain large amounts of aggregates which are difficult to disperse, so metal-base composite materials reinforced with those fibers experience great variations in mechanical strength, or plastics reinforced with those fibers are poor in surface smoothness which is one of the most important characteristics to be possessed by fiber-reinforced plastics. The potassium titanate fibers have the additional problem of high reactivity with light alloy components and metal-base composite materials reinforced with those fibers will suffer from the decrease in strength upon heat treatment. Because of these disadvantages, the prior art potassium hexatitanate fibers having a tunnel structure have not been preferred as a reinforcement for plastics and light alloys.
Another class of reinforcements conventionally known to be useful for producing composite materials are whiskers such as silicon carbide whiskers, silicon nitride whiskers, aluminum borate whiskers and potassium titanate whiskers but it is only the potassium titanate whiskers that are extensively used as reinforcements for general-purpose plastics and engineering plastics. This is because the price of the whiskers other than potassium titanate whiskers is exorbitant. Yet, compared to glass fibers and wollastonite which are extensively used as reinforcements for plastics, the price of potassium titanate whiskers is still high and this has put considerable limits on the applicability of composite materials containing potassium titanate whiskers. In order to expand the use of composite materials reinforced with potassium titanate whiskers and to upgrade the importance of those whiskers as reinforcements for plastics, it would be at least necessary to adopt new approaches such as developing potassium titanate whiskers that are as cheap as glass fibers or developing potassium titanate whiskers that can be produced at comparable costs to the prior art and which yet have higher strength.
Of the two approaches described above that are intended to produce inexpensive potassium titanate whisker reinforced plastics, the development of inexpensive potassium titanate whiskers has seen several improvements (see, :for example, Japanese Patent Public Disclosure Nos. 1615/1985 and 191019/1985). However, as regards the other approach, namely, increasing the strength of potassium titanate whiskers so as to develop a composite material that is inexpensive and that has high mechanical strength, no activities worth mention have ever been attempted.
As described above, composite materials containing the prior art potassium titanate whiskers have suffered from the problem that because of the high cost of the whiskers, the price of plastics filled with those whiskers must inevitably be set high and, as a result, the reinforced plastics have only found extremely limited utility. Under the circumstances, it has been desired to develop a novel composite material that is inexpensive and that has high strength.